1. Field of the Invention
The present invention generally relates to a deposition process for coating a substrate with an ultrasonically generated aerosol spray. More particularly, the present invention relates to a metallic foil provided with an ultrasonically generated aerosol spray. Still more particularly, the present invention provides a porous, high surface area metal oxide, metal nitride, metal carbon nitride or metal carbide coating on a conductive foil for use in a capacitor and the like.
2. Prior Art
In redox active structures, energy storage occurs during a change in the oxidation state of the metal when an ionic species from a conducting electrolyte, for example a proton, reacts with the surface or bulk of the oxide. This chemisorption is accompanied by the simultaneous incorporation of an electron into the oxide. The surface (or bulk) interaction between the electrode and the electrolyte gives rise to capacitance in the hundreds of xcexcF/sq.cm. It follows that a electrode with high specific surface area will store a significant amount of energy and will have a large specific capacitance. These electrodes are then appropriate when used as the anode and/or cathode in electrochemical capacitors or as cathodes in electrolytic capacitors, which require high specific capacitances.
Whether an anode or a cathode in an electrochemical capacitor or the cathode in an electrolytic capacitor, a capacitor electrode generally includes a substrate of a conductive metal such as titanium or tantalum provided with a semiconductive or pseudocapacitive oxide coating, nitride coating, carbon nitride coating, or carbide coating. In the case of a ruthenium oxide cathode, the coating is formed on the substrate by dissolving a ruthenium oxide precursor such as ruthenium chloride or ruthenium nitrosyl nitrate in a solvent. The solution is contacted to a substrate heated to a temperature sufficient to, for all intents and purposes, instantaneously convert the deposited precursor to a highly porous, high surface area pseudocapacitive film of ruthenium oxide provided on the substrate.
The prior art describes various methods of contacting the substrate with the semiconductive or pseudocapacitive solution, or precursor thereof. Commonly used techniques include dipping and pressurized air atomization spraying of the pseudocapacitive material onto the substrate. Capacitance values for electrodes made by dipping, pressurized air atomization spraying and sputtering are lower in specific capacitance. Sol-gel deposition is another conventional method of coating the substrate. It is exceptionally difficult to accurately control the coating morphology due to the controllability and repeatability of the various prior art techniques, which directly impacts capacitance.
Therefore, while electrochemical capacitors provide much higher energy storage densities than conventional capacitors, there is a need to further increase the energy storage capacity of such devices. One way of accomplishing this is to provide electrodes which can be manufactured with repeatably controllable morphology according to the present invention, in turn benefitting repeatably increased effective surface areas.
The present invention describes the deposition of an ultrasonically generated, aerosol spray of a pseudocapacitive metal compound or a precursor of the compound onto a heated conductive substrate. The heated substrate serves to instantaneously solidify the compound and in the case of the solution containing a precursor, convert the precursor to the pseudocapacitive metal compound provided on the substrate in a solid form. When a liquid is ultrasonically atomized, the resultant droplets are much smaller in size than those produced by a pressurized air atomizer and the like, i.e., on the order of microns and submicrons in comparison to predominately tens to hundred of microns, which results in a greater surface area coating. Therefore, the capacitance of pseudocapacitors can be further improved by using an electrode coated with an ultrasonically deposited porous film to increase the surface area of the electrodes. Additionally, depositing the aerosol onto a heated substrate results in fewer process steps, minimization of contamination of the coating by reducing process locations, increased surface area for the coating by reducing moisture absorption, and the like. The benefits result in a coated substrate that is useful as an electrode in a capacitor and the like having increased energy storage capacity.
These and other aspects of the present invention will become more apparent to those skilled in the art by reference to the following description and the appended drawings.